This invention relates to a method and apparatus for controlling the spacing between letters and other characters in a display system. In an illustrative embodiment, the invention is utilized in a display system in which characters are displayed by matrices of display elements. In a more general sense, however, the invention is applicable to any system capable of forming characters or fonts having character representations of varying width (e.g. ink jet printers).
Practical display systems employing elements such as light-emitting diodes, rotatable or "flippable" dots, or other types of display elements capable of being arranged in matrices or arrays for the forming of letters or symbols have been used for many years. Display panels including a large number of such arrays or display units are commercially available. One such system, for example, is described in U.S. Pat. No. 4,216,471.
A particularly advantageous use of such display systems is for the display of destinations of moving vehicles such as buses. Typically, such systems incorporate a plurality of display units in one or more lines, each unit of which includes a plurality of flip dots arranged in a seven-high by five-wide array or matrix (with seven rows and five columns of display elements). Each of these matrices forms a fixed-size "character display space" capable of displaying one character or symbol. For a bus destination sign, there is a constraint on the total length of the sign due to the structure of the cavity in which the sign is installed. In addition, each line of the sign must be capable of displaying a number of characters sufficient to describe most destinations, usually at least approximately 15 characters).
It is desirable to make the height of each character as great as possible, within space limitations, in order to increase legibility at a distance. However, as character height is increased, the spacing between characters must be increased to prevent the characters from seeming to run together as the distance between the observer and the sign increases.
In one typical scheme for displaying the characters on such a display system, the depiction of each character uses the entire "character display space" occupied by a 5.times.7 array of display elements. Such a sign can be difficult to read at a distance when adjacent characters are of a "block type", (e.g. M, A, N, H or A), because these characters appear to the eye to merge together at a distance. In this system, the only way to increase the spacing between characters is to increase the separation between "character display spaces" (here the physical space between each pair of 5.times.7 arrays). Unfortunately, in view of the limited total space available, this solution can reduce the number of character spaces and hence the maximum number of characters which may be displayed in one line of text.
In another prior system, only four out of the available five columns of each array are utilized for displaying each character, except for some characters which require all five columns (e.g. M, T, V, W, X). The characters which are four columns wide are all right-shifted in their respective character spaces (5.times.7 arrays) leaving the leftmost columns of the array vacant.
Display systems such as those described above include an operator control panel, a microprocessor, and a message memory such as a Programmable Read Only Memory (PROM) or an Eraseable Programmable Read Only Memory (EPROM). The message memory contains all of the possible messages which may be displayed by the system, with their characters usually coded in ASCII format. Utilizing the operator control panel, the operator may specify which of these messages is to be displayed. The ASCII data defining the characters of this message is then successively read into the microprocessor where each ASCII-encoded character is translated into signals for actuating the display elements to provide a corresponding display representation of the character (e.g. a pattern of dots for forming the corresponding character in the appropriate character space).